Editing Cancer (constellation) (section)

===Deep-sky objects===
[[File:Messier 44 2018.jpg|alt=Image of Messier 44 (the Beehive Cluster)|thumb|Messier 44 (the Beehive Cluster)]]
Cancer is best known among stargazers as the home of [[Beehive Cluster|Praesepe]] (Messier 44), an [[open cluster]] also called the ''Beehive Cluster'', located right in the centre of the constellation. Located about 590 light-years from Earth, it is one of the nearest open clusters to our Solar System. M 44 contains about 50 stars, the brightest of which are of the sixth magnitude. Epsilon Cancri is the brightest member at magnitude 6.3. Praesepe is also one of the larger open clusters visible; it has an area of 1.5 square degrees, or three times the size of the full Moon.<ref name="ridpath"/> It is most easily observed when Cancer is high in the sky. North of the Equator, this period stretches from February to May. 

[[Ptolemy]] described the Beehive Cluster as "the nebulous mass in the breast of Cancer." It was one of the first objects [[Galileo]] observed with his telescope in 1609, spotting 40 stars in the cluster. Today, there are about 1010 high-probability members, most of them (68 percent) red dwarfs. The Greeks and Romans identified the nebulous object as a manger from which two donkeys, represented by the neighbouring stars [1213] Asellus Borealis and [1210] Asellus Australis, were eating. The stars represent the donkeys that the god [[Dionysus]] and his tutor [[Silenus]] rode in the war against the [[Titan (mythology)|Titan]]s. The ancient Chinese interpreted the object as a ghost or demon riding in a carriage, calling it a "cloud of pollen blown from under willow catkins."{{cn|date=March 2024}}

The smaller, denser open cluster [[Messier 67]] can also be found in Cancer, 2600 light-years from Earth. It has an area of approximately 0.5 square degrees, the size of the full Moon. It contains approximately 200 stars, the brightest of which are of the tenth magnitude.<ref name="ridpath"/>

[[QSO J0842+1835]] is a [[quasar]] used to measure the [[speed of gravity]] in [[VLBI]] experiment conducted by [[Edward Fomalont]] and [[Sergei Kopeikin]] in September 2002.

[[OJ 287]] is a [[BL Lacertae object]] located 3.5 [[1000000000 (number)|billion]] [[light years]] away that has produced quasi-periodic optical outbursts going back approximately 120 years, as  first apparent on photographic plates from 1891.  It was first detected at radio wavelengths during the course of the [[Ohio Sky Survey]]. Its central [[supermassive black hole]] is [[List of most massive black holes|among the largest known]], with a mass of 18 billion [[solar masses]],<ref>{{Cite journal | last1 = Valtonen | first1 = M. J. | last2 = Lehto | first2 = H. J. | last3 = Nilsson | first3 = K. | last4 = Heidt | first4 = J. | last5 = Takalo | first5 = L. O. | last6 = Sillanpää | first6 = A. | last7 = Villforth | first7 = C. | last8 = Kidger | first8 = M. | last9 = Poyner | first9 = G. | last10 = Pursimo | doi = 10.1038/nature06896 | first10 = T. | last11 = Zola | first11 = S. | last12 = Wu | first12 = J. -H. | last13 = Zhou | first13 = X. | last14 = Sadakane | first14 = K. | last15 = Drozdz | first15 = M. | last16 = Koziel | first16 = D. | last17 = Marchev | first17 = D. | last18 = Ogloza | first18 = W. | last19 = Porowski | first19 = C. | last20 = Siwak | first20 = M. | last21 = Stachowski | first21 = G. | last22 = Winiarski | first22 = M. | last23 = Hentunen | first23 = V. -P. | last24 = Nissinen | first24 = M. | last25 = Liakos | first25 = A. | last26 = Dogru | first26 = S. | title = A massive binary black-hole system in OJ 287 and a test of general relativity | journal = Nature | volume = 452 | issue = 7189 | pages = 851–853 | year = 2008 | pmid = 18421348 | url = http://astrophysics.rit.edu/pastjclub/nature06896.pdf | arxiv = 0809.1280 | bibcode = 2008Natur.452..851V | s2cid = 4412396 | access-date = 1 September 2015 | archive-date = 8 September 2015 | archive-url = https://web.archive.org/web/20150908193115/http://astrophysics.rit.edu/pastjclub/nature06896.pdf | url-status = dead }}</ref> more than six times the value calculated for the previous largest object.<ref name="newscientist">{{cite news| url=https://www.newscientist.com/article/dn13166-biggest-black-hole-in-the-cosmos-discovered.html|title= Biggest black hole in the cosmos discovered| date=10 January 2008| publisher= NewScientist.com news service| first= David| last= Shiga}}</ref>